Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 15 to 20 minutes. Drain water from pot and mash potatoes with a potato masher or potato ricer.
Heat buttermilk and butter together in a saucepan over medium-low heat until butter is melted and mixture is smooth, about 2 minutes. Stir buttermilk mixture and chives into potatoes until smooth; season with salt.